Cheated on much?
Look dude be logical. What you are kind of saying is that if no one was ever willing to hook-up with someone in a relationship, there would be no cheating. And thats true. But thats kind of asking the people on the outside to police your relationship for you. Thats not their respnsibility.
You are preaching loyalty by default. Someone that doesn't cheat is loyal. Someone that has no opportunity to cheat is also loyal.
Thats not true loyalty. Loyalty is correct action in the face of temptation. Without temptation there is no loyalty. IF the only reason your SO isn't screwing others is because he/she doesn't have the chance .. . well that might not be the best basis to have a relationship.
Just like fear. If you don't feel fear, you can't be courageous. Its only in acting despite your fear that you show courage.